1. Field of the Invention
The present invention generally relates to an electrophotographic image forming apparatus utilizing a xerographic process and, more specifically, to a toner measuring mechanism used in the electrophotographic image forming apparatus for measuring the quantity of toner deposited on an electrostatic latent image carrier.
2. Description of the Related Art
In an electrophotographic image forming apparatus utilizing the Carson or xerographic process such as, for example, an electrophotographic copying machine or a laser printer, a toner measuring mechanism comprising a photoelectric toner sensor assembly is generally utilized to measure the quantity of toner supplied onto a photosensitive medium such as a photoreceptor drum on a trial basis. Any indication of the quantity of toner deposit provided by the photoelectric toner sensor assembly is subsequently utilized to effect various controls necessary to stabilize the density of toner used during an actual development, the bias voltage applied during the actual development, the voltage applied to electrostatic chargers, the intensity of light used during the actual development and so on for eventually accomplishing a favorable image reproduction.
An example of the photoelectric toner sensor assembly used for that purpose and operable with a color toner is generally identified by 100 in FIG. 6. The photoelectric toner sensor assembly 100 comprises a light emitting diode (LED) 101 and a light receiving element or photodiode 102 arranged in a side-by-side fashion with each other and spaced a distance outwardly from a photosensitive surface of the photoreceptor drum 110. Specifically, the light receiving element 102 is so positioned relative to the light emitting diode 101 as to receive a diffused component of rays of light from the light emitting diode 101 which has undergone a diffuse reflection from a layer of color toner deposited on the photosensitive surface of the photoreceptor drum 110. The light emitting and receiving elements 101 and 102 have their own optical axes lying on a common plane and are so positioned that the common plane containing the respective optical axes of the light emitting and receiving elements 101 and 102 can either coincide with or lie parallel to a plane containing the axis of rotation of the photoreceptor drum 110.
As is well known to those skilled in the art, the rays of light projected from the light emitting diode 101 and subsequently reflected by the toner deposit on the photoreceptor drum contain a specular reflecting component which undergoes a specular reflection and a diffused reflecting component which undergoes a diffuse reflection. On the other hand, when it comes to the use of the color toner in the image forming apparatus, an increase of the quantity of the color toner deposit on the photoreceptor drum will bring about little reduction in amount of the rays of light undergoing the specular reflection and will rather result in a steep gradient of increase in amount of the rays of light undergoing the diffuse reflection. In view of this, the photoelectric toner sensor assembly is of a design detecting the diffuse reflection so that an accurate indication of the quantity of the toner deposit can be obtained.
The U.S. Pat. No. 4,617,245, issued Oct. 14, 1986, discloses the use on a photoreceptor medium of a means for suppressing interference fringes known as Moire fringes resulting from interference of light which often occurs where an electrostatic latent image is formed by causing a laser beam to scan the photosensitive surface. Specifically, while the disclosed photoreceptor medium is in the form of a drum comprising an electroconductive cylinder and a photosensitive layer formed on an outer peripheral surface of the cylinder, the fringe suppressing means comprises a multiple of circumferentially extending and juxtaposed grooves formed on the outer peripheral surface of the electroconductive cylinder. Each of those grooves is described having a groove face (or "a tapered reflective surface" according to a terminology used therein) of a generally U-shape, V-shape, trapezoidal or semi-ellipsoidal shape in section while leaving a circumferentially extending ridge (or "a linear projection" according to the same) defined between each neighboring grooves. This patent also discloses that the grooves for the suppression of the interference fringes are of a depth (or "a taper height" according to the same) equal to half the wavelength of incident light used during an image exposure, preferably 100 .mu.m or less and, more preferably, within the range of 0.3 to 30 .mu.m, and of a width within the range of 10 to 500 .mu.m and spaced at a pitch of 1,000 .mu.m or less.
This patent describes that the grooves referred to above may not be always limited to those juxtaposed with each other, but may be parts corresponding to turns of a spiral groove extending about the axis of rotation of the photoreceptor drum.
During the course of development of the present invention, the inventor has found a problem occurring when, as one of the methods of arranging the photoelectric sensor assembly relative to the photoreceptor drum of a type provided with the fringe suppressing means, the photoelectric toner sensor assembly is arranged such as shown in FIG. 6. Specifically, in the system shown in FIG. 6, the employment of the fringe suppressing means is liable to increase the diffuse reflection from the outer peripheral surface of the photoreceptor drum 110. Therefore, the photoelectric toner sensor assembly 100 tends to provide an output of such a characteristic as shown in FIG. 7 wherein the voltage of the sensor output is relatively high when the quantity of toner deposited on the photosensitive layer is zero, that is, when the photosensitive layer is exposed bare. This results in a reduction in signal-to-noise ratio during the measurement of the quantity of the toner deposit, accompanied by a reduction in amount of change of the sensor output voltage. In view of the foregoing, a relatively large error tends to occur in the measurement of the quantity of the toner deposit and, therefore, it is generally considered difficult to accomplish a favorable image stabilizing control.
It is pointed out that the inventor of the present invention has no knowledge of any prior art literature which is addressed to the system wherein the photoreceptor medium of the type having the fringe suppressing means formed thereon is actually used in combination with a specific arrangement of the photoelectric toner sensor assembly such as discussed with reference to FIG. 6.